opt-in

To volutarily sign up for something, this term has been used increasingly during the Internet age, normally to describe mailing lists or marketing campaigns. The recipients opt in to the mailing list and no spam is sent to those who do not opt in. This term is the opposite of opt out

Unfortunately, spammers lately seem to have confused opt-out with opt in. They claim in their junk mail that they are an
opt in mailing list, even though you've never heard of them
before but you're on their list.

Sometimes they even prefix their letter with claims
that you subscribed to their list and actually
asked for it.

I doubt a technical solution will ever fix the spam problem.
We'll need legislation like the Telephone Consumer Protection Act which has specific wording to the effect of "prior business relationship", although this is probably too simple. For example, I don't mind if a linux vendor finds
my linux web page and sends me a job offer or even a product announcement. I do mind when a diet hoax spammer finds my thin client page.... but that rant belongs in a different node. Perhaps it is enough that the FTC is beginning to prosecute fraudulent email scams.